Why is double spending impossible

Even if it takes up to 3 minutes, until a transaction is in the blockchain, any transaction is valid immediately. If anybody would try to duplicate a transaction within the block period timeframe, it would be still rejected by the mempool. Each node has a copy of that mempool and before entering the blockchain the time when a transaction has entered it will be compared with others. In case there is a difference, the CVN that creates the next block will confirm the one it received first. The second one will be stripped off all mempools once the confirming block is received. (txn-mempool-conflict - Transaction rejected by network rules).

To sum up: FairCoin does not support instant transaction confirmation, but we have a very strong additional mechanism to avoid any double spending. Even if we could not think of a hypothetical case of double spending, we recommend to wait at least two confirmations if you transfer big amounts. Watch the block explorer to see your transactions in the unmutable blockchain.